Reduction of somatostatin receptors in rat hippocampus by treatment with 5,7-dihydroxytryptamine

Anxiolytic and antidepressant actions of somatostatin: the role of sst2 and sst3 receptors

RATIONALE AND OBJECTIVES

Somatostatin is a cyclic polypeptide that inhibits the release of a variety of regulatory hormones (e.g., growth hormone, insulin, glucagon, and thyrotropin). Somatostatin is also widely distributed within the central nervous system (CNS), acting both as a neurotransmitter and as a neuromodulator. Recently, we showed that intracerebroventricular (i.c.v.) administration of somatostatin reduced anxiety-like and depression-like behaviors in animal models. The somatostatin receptor subtypes that are involved in these behavioral effects, however, have not been investigated. In the CNS, the neurotransmitter actions of somatostatin are mediated through five G-protein coupled receptors (sst1 to sst5).

MATERIALS AND METHODS

We examined the behavioral effects of i.c.v. microinfusions of different doses of selective agonists of each of the five somatostatin receptor subtypes. Their behavioral effects were assessed in the elevated plus-maze and the forced swim apparatus, rodent models of anxiolytic and antidepressant drug effects, respectively.

RESULTS

Anxiety-like behavior was reduced following i.c.v. infusions of a selective sst2 receptor agonist, but not after infusions of the other four receptor agonists. An antidepressant-like effect was observed following infusions of either sst2 or sst3 agonists.

CONCLUSIONS

The results add to our nascent understanding of the role of somatostatin in anxiety- and depression-like behavior and suggest a clinical role for somatostatin agonists for the simultaneous treatment of anxiety and depression, which are often comorbid.

Antidepressants influence somatostatin levels and receptor pharmacology in brain

This study investigated how the administration (acute and chronic) of the antidepressants citalopram and desmethylimipramine (DMI) influences somatostatin (somatotropin release inhibitory factor, SRIF) levels and SRIF receptor density (sst(1-5)) in rat brain. Animals received either of the following treatments: (1) saline for 21 days (control group), (2) saline for 20 days and citalopram or DMI for 1 day (citalopram or DMI acute groups), (3) citalopram or DMI for 21 days (citalopram or DMI chronic groups). Somatostatin levels were determined by radioimmunoassay. [(125)I]LTT SRIF-28 binding in the absence (labeling of sst(1-5)) or presence of 3 nM MK678 (labeling of sst(1/4)) and [(125)I]Tyr(3) octreotide (labeling of sst(2/5)) binding with subsequent autoradiography was performed in brains of rats treated with both antidepressants. Somatostatin levels were increased after citalopram, but not DMI administration, in the caudate-putamen, hippocampus, nucleus accumbens, and prefrontal cortex. Autoradiography studies illustrated a significant decrease in receptor density in the superficial and deep layers of frontal cortex (sst(2)), as well as a significant increase in the CA1 (sst(1/4)) hippocampal field in brains of chronically citalopram-treated animals. DMI administration increased sst(1/4) receptors levels in the CA1 hippocampal region. These results suggest that citalopram and to a lesser extent DMI influence the function of the somatostatin system in brain regions involved in the emotional, motivational, and cognitive aspects of behavior.

Changes in brain somatostatin in memory-deficient rats: comparison with cholinergic markers

To clarify the functional role of the brain somatostatinergic system in cognitive processes, changes in the performance in passive avoidance and water maze tasks and in brain somatostatin contents were comparatively investigated in young Fischer rats subjected to brain cholinergic and somatostatinergic depletion, and in aged Fischer rats. Lesioning of the nucleus basalis magnocellularis and administration of cysteamine (200 mg/kg, s.c.), a depletor of somatostatin, resulted in significant deficits in passive avoidance, but complete transection of the fimbria-fornix hardly affected the performance in the task. When cognitive performance was assessed in the Morris water maze, lesions of the nucleus basalis magnocellularis and the fimbria-fornix, and administration of cysteamine, significantly impaired the acquisition of navigatory spatial memories of rats. On the other hand, aged rats (24-27 months) showed severe impairments of memory acquisition in both tasks. Neurochemistry measurements showed that lesions of the nucleus basalis magnocellularis produced a selective reduction both in the cortical cholinergic marker choline acetyltransferase and in striatal somatostatin level, whereas lesioning of the fimbria-fornix caused a marked loss of choline acetyltransferase in the hippocampus and posterior cortex, and a significant reduction in hippocampal somatostatin. On the other hand, treatment with cysteamine significantly reduced the contents of somatostatin in all the brain regions examined, but minimally affected choline acetyltransferase activity. However, significant reduction in the striatal choline acetyltransferase activity and elevation in somatostatin content in the frontal cortex were found in aged rats compared with young rats. Taken together, these results strongly suggest that changes in the brain somatostatinergic transmission are involved in the cognitive deficits in the experimental animal models of dementia presently employed. Furthermore, the present comparative study further implies that there are differences in the relative involvement of the cholinergic and somatostatinergic systems in the performance of rats on two different tests of mnemonic function.